The present invention relates to a power transmission system for a full-time four-wheel drive vehicle having a mechanism for absorbing the difference between the speeds of the front and rear wheels.
When a vehicle negotiates corners, the front wheels run through an arc of greater radius than that of the rear wheels and therefore tend to rotate faster than the rear wheels. In a four-wheel drive vehicle without a device for absorbing the difference between the speeds of the front and rear wheels, the difference causes the braking of the vehicle, known as "tight corner braking". In order to prevent such a braking phenomenon, there has been proposed a power transmission system having a double overrunning clutch system for absorbing the difference of speeds of front and rear wheels, and a system for locking the system for preventing at least one of wheels from slipping on slippery roads. The overrunning clutch system comprises forward and reverse overrunning clutches each in the form of a freewheel and a sleeve for selecting forward driving mode, reverse driving mode or a directly connected four-wheel driving mode for the locking of the system. These modes can be selected by engaging toothings of the sleeve with toothings of one of the clutches or with toothings of both clutches.
In such a system, the selecting operation of the clutches must be swiftly completed. However, if each toothing is of a shape having a rectangular tooth crest, toothings of the sleeve can collide with toothings of the selected clutches, when the sleeve approaches the clutch. Thus, a further movement of the sleeve is stopped. Accordingly, proper engagement of the sleeve and the clutch is impossible unless their relative positions are changed by an external force.
If the corners of each toothing are chamfered to form a tapered crest as in toothings of a synchronizer of a transmission, a chamfer of the selected clutch possibly abuts an improper side of the chamfer of the sleeve. Namely, when one of the chamfers of the sleeve abuts against one of the chamfers of the clutch, the clutch is forced to rotate in one direction. If the direction is the locking direction of the clutch, the clutch cannot rotate. Thus, the sleeve also stops shifting.